This invention relates to oscillators, for example, a voltage controlled oscillator circuit employing a negative resistance technique.
Voltage controlled oscillator (VCO) circuits are well known in the art and are utilized in a number of applications. A VCO circuit is typically responsive to a control signal that is varied to adjust the frequency of an output signal of the VCO circuit.
An oscillator circuit may be converted to a psuedo voltage controlled oscillator circuit wherein the oscillation frequency of the oscillator circuit is determined by an external inductor (L) and capacitor (C) tank circuit. Further, by utilizing an external varactor to function as the capacitor of the external LC tank circuit, the capacitance of the varactor can be varied by varying the voltage applied to the varactor. As a result, the resonant frequency of the external LC tank circuit is correspondingly varied thereby varying the oscillation frequency of the oscillator circuit. However, a varactor is very expensive. Further, the varactor increases the phase noise of the oscillation circuit.
A prior art voltage controlled oscillator circuit that does not utilize a varactor is fully disclosed in an article entitled "Varactorless VCOs: Transistors go it alone", on pages 137-142 of the June 1984 issue of Microwaves & RF. Referring to FIG. 2 of the article, a modified Colpitts oscillator is shown. In particular, an adjustable voltage is applied to the base of a transistor to vary the base-emitter voltage and emitter current of the transistor. This, in turn, varies the emitter-base capacitance of the transistor. Further, by varying the emitter-base capacitance of the transistor, the capacitance of the LC tank circuit is varied and the oscillation frequency of the oscillator circuit is varied. However, since the adjustable voltage is applied to the base of the transistor, the adjustable voltage has a prominent effect on the bias of the transistor. For example, as the adjustable voltage is lessened, the voltage swing of the output signal of the oscillator circuit is reduced since the transistor is not properly biased. In addition, the output signal of the oscillator circuit is not symmetrical.
Hence, there exists a need to provide an improved VCO circuit having voltage controlled tuning capability while maintaining symmetry and adequate voltage of the output signal of the VCO circuit.